1. Field of the Invention
The present invention relates to an air-conditioning system comprising a variable displacement type compressor for compressing and discharging a refrigerant while at the same time changing the discharge capacity by changing a control pressure, and a cooling circuit and a heating circuit sharing the use of the variable displacement type compressor, in which the cooling circuit or the heating circuit is selectively used.
2. Description of the Related Art
An air-conditioning system of this type is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 7-19630. When the cooling circuit is used, the refrigerant discharged from the compressor recirculates to the compressor through a condenser, an expansion valve and an evaporator. When the heating circuit is in use, on the other hand, the refrigerant discharged from the compressor recirculates to the compressor through an evaporator and a pressure reducing device on a first bypass. The operation of the cooling circuit and the heating circuit is switched by operating a switching valve.
When the heating circuit is used, the condenser is isolated, and therefore the discharge pressure is determined by the balance between the discharge amount of the compressor and the flow rate in the pressure reducing device on the bypass. As a result, the discharge pressure of the compressor is liable to be higher when the heating circuit is in use than when the cooling circuit is in use. Thus, when the heating circuit is in use, the discharge pressure often rises to an abnormally high level. In view of this, the conventional system disclosed in Kokai No. 7-19630 comprises a pressure relief valve interposed on a second bypass connecting the cooling circuit and the heating circuit. In the case where the discharge pressure rises to an abnormally high level while the heating circuit is being used, the pressure relief valve is opened to release the refrigerant from the heating circuit into the condenser.
Since the refrigerant is released into the condenser from the heating circuit each time the discharge pressure rises to an abnormally high level when the heating circuit is used, however, the refrigerant in the heating circuit is reduced in amount to such an extent that the heating capacity becomes insufficient. Also, the refrigerant is released wastefully to the condenser at the sacrifice of the work required on the part of the compressor to increase the pressure, thereby leading to a low energy efficiency.
In the case where a variable displacement type compressor disclosed in Kokai No. 11-180138, i.e. in the case where a variable displacement type compressor with an adjustable suction pressure setting is used as a compressor shared by the cooling circuit and the heating circuit, the situation may occur in which the air-conditioning system cannot perform the heating function. Specifically, the capacity control valve used with the variable displacement type compressor disclosed in Kokai No. 11-180138 functions to reduce the discharge capacity to such an extent as to reduce the pressure setting described above. Once the atmospheric temperature drops to such a level that the saturated pressure of the refrigerant drops below the range of the pressure setting, therefore, the discharge capacity cannot be increased and the air-conditioning system cannot perform the heating function.
The object of this invention is to provide an air-conditioning system comprising a variable displacement type compressor, and a cooling circuit and a heating circuit selectively sharing the variable displacement type compressor, in which the heating function is always effectively and efficiently performed on the one hand and a reduction in the cooling and heating functions is avoided on the other hand.
According to the invention, there is provided an air-conditioning system comprising a variable displacement type compressor for compressing and discharging the refrigerant while at the same time changing the discharge capacity by changing the control pressure, a cooling circuit and a heating circuit selectively sharing the use of the variable displacement type compressor, and capacity control means for controlling the discharge capacity of the variable displacement type compressor based on the differential pressure between the pressure in the discharge pressure area and the pressure in the suction pressure area or between the pressure in the discharge pressure area and the control pressure, wherein the capacity control means controls both the discharge capacity for circulating the refrigerant in the cooling circuit and the discharge capacity for circulating the refrigerant in the heating circuit.
By controlling the discharge capacity based on the differential pressure between the pressure in the discharge pressure area and the suction pressure area or between the pressure in the discharge pressure area and the control pressure, the discharge capacity can be controlled upward and therefore the air-conditioning system can perform the heating function even when the temperature is very low.
The present invention may be more fully understood from the description of a preferred embodiment set forth below, together with the accompanying drawings.